e1087. try/catch语句

The try/catch statement encloses some code and is used to handle errors and exceptions that might occur in that code. Here is the general syntax of the try/catch statement:

    try {
        body-code
    } catch (exception-classname variable-name) {
        handler-code
    }

The try/catch statement has four parts. The body-code contains code that might throw the exception that we want to handle. The exception-classname is the class name of the exception we want to handle. The variable-name specifies a name for a variable that will hold the exception object if the exception occurs. Finally, the handler-code contains the code to execute if the exception occurs. After the handler-code executes, execution of the thread continues after the try/catch statement. Here is an example of code that tries to create a file in a non-existent directory which results in an IOException.

    String filename = "/nosuchdir/myfilename";
    try {
        // Create the file
        new File(filename).createNewFile();
    } catch (IOException e) {
        // Print out the exception that occurred
        System.out.println("Unable to create "+filename+": "+e.getMessage());
    }
    // Execution continues here after the IOException handler is executed

Here's the output:

    Unable to create /nosuchdir/myfilename: The system cannot find the path specified

It is possible to specify more than one exception handler in a try/catch statement. When an exception occurs, each handler is checked in order (i.e. top-down) and the handler that first matches is executed. The following example registers a name at a website. The example code can throw two kinds of exceptions -- one if the URL is invalid and one if the web page is not accessible. This example uses an invalid URL which results in a MalformedURLException.

    // This URL string is deliberately missing the http: protocol to cause an exception
    String urlStr = "xeo.com:90/register.jsp?name=joe";
    try {
        // Get the image
        URL url = new URL(urlStr);
        InputStream is = url.openStream();
        is.close();
    } catch (MalformedURLException e) {
        // Print out the exception that occurred
        System.out.println("Invalid URL "+urlStr+": "+e.getMessage());
    } catch (IOException e) {
        // Print out the exception that occurred
        System.out.println("Unable to execute "+urlStr+": "+e.getMessage());
    }

Here's the output:

    Invalid URL xeo.com:90/register.jsp?name=joe: no protocol: xeo.com:90/register.jsp?name=joe

Here is the same example with a valid URL. The URL refers to a non-existant web page which results in a IOException.

    urlStr = "http://xeo.com:90/register.jsp?name=joe";
    try {
        URL url = new URL(urlStr);
        InputStream is = url.openStream();
        is.close();
    } catch (MalformedURLException e) {
        // Print out the exception that occurred
        System.out.println("Invalid URL "+urlStr+": "+e.getMessage());
    } catch (IOException e) {
        // Print out the exception that occurred
        System.out.println("Unable to execute "+urlStr+": "+e.getMessage());
    }

Here's the output:

    Unable to execute http://xeo.com:90/register.jsp?name=joe: Connection refused: connect

When an exception occurs, the exception is compared with the specified exception class name using the instanceof comparator. This means that if the handler specifies a class name E, then any exception whose class either equals E or is a subclass of E, matches the handler. The following example catches either MalformedURLException or IOException using Exception since both exceptions are subclasses of Exception.

    urlStr = "http://xeo.com:90/register.jsp?name=joe";
    try {
        URL url = new URL(urlStr);
        InputStream is = url.openStream();
        is.close();
    } catch (Exception e) {
        // Print out the exception that occurred
        System.out.println(urlStr+": "+e.getMessage());
    }

Here's the output:

    http://xeo.com:90/register.jsp?name=joe: Connection refused: connect

If an exception occurs and does not match any of the handlers, the exception is passed to any enclosing try/catch statements and up the call chain until it is caught by some handler. If the exception is not handled by any handler, it becomes an uncaught exception and the thread is immediately terminated. In the following example, a NullPointerException is thrown, which is not caught by MalformedURLException and so becomes an uncaught exception.

    // This string is deliberately set to null to cause an exception
    urlStr = null;
    try {
        int len = urlStr.length(); // Causes a NullPointerException
        URL url = new URL(urlStr);
    } catch (MalformedURLException e) {
        // Print out the exception that occurred
        System.out.println("Invalid URL "+urlStr+": "+e.getMessage());
    }

The result of an uncaught exception is a stack trace that identifies the line of code that threw the exception. The output looks something like:

    Exception in thread "main" java.lang.NullPointerException
            at MyClass.mymethod(MyClass.java:162)

 

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